Lyme Disease -- United States, 1996

Lyme disease (LD) is caused by the tickborne spirochete
Borrelia burgdorferi sensu lato and is the most common vectorborne
disease in the United States. Surveillance for LD was initiated by
CDC in 1982, and the Council of State and Territorial
Epidemiologists designated it a nationally notifiable disease in
January 1991. For surveillance purposes, LD is defined as the
presence of an erythema migrans rash greater than or equal to 5 cm
in diameter or laboratory confirmation of infection with evidence
of at least one manifestation of musculoskeletal, neurologic, or
cardiovascular disease (1). This report summarizes the provisional
number of cases of LD reported to CDC during 1996 and indicates
that the number of cases reported to CDC was a record high.

In 1996, a total of 16,461 cases of LD were reported to CDC
by 45 states and the District of Columbia (overall incidence: 6.2
per 100,000 population *), representing a 41% increase from the
11,700 cases reported in 1995 and a 26% increase from the 13,043
cases reported in 1994 (Figure_1). As in previous years, most
cases were reported from the Mid-Atlantic, Northeast, and North
Central regions (Table_1). Eight states reported LD incidences
that were higher than the overall national rate (Connecticut,
94.8; Rhode Island, 53.9; New York, 29.2; New Jersey, 27.4;
Delaware, 23.9; Pennsylvania, 23.3; Maryland, 8.8; and Wisconsin,
7.7); these states accounted for 14,959 (91%) of the nationally
reported cases. In 1996, zero cases were reported from five states
(Alaska, Arizona, Colorado, Montana, and South Dakota).

Eighty-seven counties each reporting greater than or equal to
20 cases accounted for 89% of all reported cases. Reported
incidences were greater than 100 per 100,000 ** in 18 counties in
Connecticut, Maryland, Massachusetts, North Carolina, New Jersey,
New York, Pennsylvania, Rhode Island, and Wisconsin; the highest
reported county-specific incidence (1247.5 per 100,000) was in
Nantucket County, Massachusetts (Figure_2). From 1995 to 1996,
a
total of 28 states reported increases in the number of cases, 16
states reported decreases, and seven states reported no change.
Approximately 90% of the total increase in reported cases in 1996
occurred in five states (Connecticut, New Jersey, New York,
Pennsylvania, and Rhode Island) where average annual LD incidence
rates had exceeded the national average for the previous 5 years
combined.

Of 5298 cases for which information was available, 217 (4%)
were reported as having been acquired outside of the United
States, and 156 (3%) cases were reported as having been acquired
in the United States but outside of the reporting state. The
highest proportions of cases occurred among persons aged 0-14
years (3784 {23%}) and adults aged 40-79 years (7694 {47%}). Of
16,422 cases for which sex was reported, 8634 (53%) were male.

Editorial Note

Editorial Note: LD continues to be an important emerging
infection: geographic spread within states with endemic disease
and intensified transmission of the LD spirochete in established
foci of infection have been associated with increased numbers of
reported cases in the United States. In the eastern United States,
the patterns of human LD cases reflect the geographic distribution
of Ixodes scapularis, also known as the black-legged or deer tick
(2,3). Substantial annual fluctuations since 1992 in the number of
reported cases in several northeastern states with endemic disease
have been attributed, in part, to variations in I. scapularis
density (4,5). The principal vector in western coastal states is

pacificus (the western black-legged tick). LD also is
transmitted by Ixodes spp. in Canada and in temperate areas of
Eurasia, including Europe, Russia, northern People's Republic of
China, and Japan (6).

Increases in reported LD cases in 1996 were limited to
certain counties in some states, consistent with focal differences
in the distribution and density of the tick vector. In both
Connecticut and Rhode Island, the numbers of reported cases of LD
increased statewide, although increases were greatest in coastal
counties. In both states, this increase was associated with
increased population densities of I. scapularis (K. Stafford,
Connecticut Agricultural Experiment Station, and T. Mather,
University of Rhode Island, personal communications, 1997). In New
York, the greatest increases occurred in Dutchess County, where
reported cases of LD nearly doubled from 1995 (918) to 1996
(1832). Because an LD vaccine trial was being conducted in the
area, some of this increase may have resulted from heightened
awareness and reporting of LD. The number of reported cases was
stable in other counties of New York with endemic disease,
including Putnam, Suffolk, and Westchester counties. In New
Jersey, eight counties with active surveillance reported higher
rates than the remaining counties with passive surveillance
systems.

Since 1991, state health departments in regions with endemic
disease have been expanding their use of laboratory testing for
assisting in LD surveillance. A positive laboratory result is
required for reporting of persons with systemic manifestations of
LD but is not required for persons with an erythema migrans rash
greater than or equal to 5 cm in diameter (i.e., early LD). Since
August 1995, when CDC published recommendations for standardized
two-step (enzyme immunoassay and Western immunoblot)
serodiagnostic testing for LD (7), states have reported a shift
toward use of the recommended two-step method in diagnostic
laboratories. The impact of these changes in laboratory methods on
LD surveillance is unknown.

The increase in reported LD cases in 1996 probably represents
a combination of increased tick density, enhanced health-care
provider awareness and reporting, and improved laboratory
surveillance. In addition, case reporting has been enhanced
through the availability of CDC resources for LD surveillance in
eight states (Connecticut, Michigan, Minnesota, New Jersey, New
York, Oregon, Rhode Island, and West Virginia).

Most LD cases respond well to appropriate antibiotic therapy;
drugs of choice include amoxicillin, doxycycline, and ceftriaxone
(8). Vaccines to prevent LD are under evaluation but are not yet
available. Personal protection methods recommended for preventing
cases of LD and other tickborne diseases (e.g., babesiosis,
ehrlichiosis, and Rocky Mountain spotted fever) include wearing
light-colored clothing (to more readily detect ticks), tucking
long pants into socks, using insect repellents and acaricides
according to label directions, and performing tick checks at least
daily. The use of environmental modifications to residential
properties (e.g., application of insecticides, use of deer
fencing, and removal of leaf litter) also may help prevent LD.

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